Inkjet printers use at least one printhead provided with a plurality of nozzles, from which ink droplets are fired or ejected onto the media; the printer controls the firing of ink from the nozzles such as to create on the media a pattern of dots corresponding to the desired image, plot, drawing, etc.
Color printers are typically provided with several printheads, for example one for each of the primary colors cyan, yellow, magenta (CYM) and black (K). The printheads may be mounted on a carriage that reciprocates in successive passes above the media along a scan direction, with the nozzles firing droplets of ink as the printhead moves across the media; after each printing pass of the printheads, the media is advanced in a media advance direction, at right angles to the scan direction, such that a plot is formed on the media in successive passes of the printheads.
The nozzles are typically arranged in one or more linear arrays, extending in the media advance direction; the length of the nozzle arrays is usually referred to as the swath height, and it corresponds to the maximum pattern of ink that can be laid down on the media in a single pass.
Such a printer can operate according to several different print modes: in a single-pass print mode, after each printing pass the media is advanced a distance equal to the printhead swath height, such that each pass forms on the media a complete strip of the image; on the contrary, in a multi-pass print mode the media only advances a fraction of the swath height after each printing pass of the printheads, for example ½, ¼ or ⅛ of the swath height, and each strip of the image to be printed is formed in successive and partially overlapping passes. Multi-pass print modes are typically used for obtaining higher quality plots.
Printing can be unidirectional, i.e. the printheads only print when travelling in one direction along the scan axis, and then they travel in a return pass without printing, or it can be bidirectional, i.e. the printheads print when travelling in a “forward pass” and also when travelling in a “return pass”, the media being advanced after each pass.
A printer can have a service station at least at one end of the path of travel of the printheads along the scan axis, in order to maintain the proper functioning conditions of the printheads, for example by performing a number of servicing operations such as wiping and spitting ink, or by capping the nozzles when not in use.
Maintenance operations may be carried out between print jobs; however, it is also known to perform some servicing operations while printing, i.e. during a print job. This may be done for example to avoid or reduce the drying of ink which is in position to be fired from the nozzles, due to the exposure to air during a print job, i.e. when the nozzles are not capped; this effect is known as “decap”, and it may cause a loss in the quality of the printed plot. “Slewing decap” is related to the current of air created by the printheads as they travel or slew along the scan axis, which increases the negative effects on the drying of the ink in the nozzles. Pigmented inks are particularly sensitive to idle times.
Typical defects of the first drops ejected after the nozzles have been idle a period of time are related to higher colorant concentration, poor drop shape, lower drop weight and/or velocity, drop placement errors, etc. These defects can cause image quality problems such as color discontinuities or a high level of grain.
Thus, to avoid or reduce the effects of decap and slewing decap, a printhead can be serviced during printing, for example by causing the printhead to travel to the service station between two printing passes and spitting ink through the nozzles, if the nozzles of the printhead exceed a predetermined time without firing ink drops on the media (for example as a result of the features of the plot being printed). Spit of a printhead during printing has been referred to as “flying spit” or “spit on the fly”.
Commonly assigned U.S. Pat. No. 6,554,392 discloses a printing apparatus with a main service station and an auxiliary service station, one at each end of the scan axis, and a printing method that comprises spitting a printhead either in the main or in the auxiliary service stations before printing each pass, if the time elapsed since the printhead was refreshed, either by printing or by a flying spit, exceeds a threshold value.
This improves the printing quality by avoiding or reducing the effect of decap; however, the spitting operation in the auxiliary service station entails the need for the carriage to overtravel a distance from the end of the plot to this service station, which is located at the end of the scan axis, and this may reduce the throughput of the printing operation.
Overtravel of the carriage to the auxiliary service station is significantly greater than overtravel to the main service station, since the media is always loaded in the printer adjacent the side where the main service station is arranged, and in many cases the media being printed is not as wide as the maximum width the printer can handle, so the auxiliary service station is remote from the edge of the media. The loss of throughput is thus especially severe when the media and the plot being printed are substantially narrower that the maximum media width of the printer.